Because you simply cannot ignore it. It is just an amazing tool to have
- This gives you WYSWYG
- Takes care of the dynamically loaded controls with data while designing
- Data bound WinJS controls are not visible in Visual Studio unlike ASP.NET
- You can also view and design for different layouts like snapping etc.
- Helps you check resolution while designing.
- It supports live DOM
- It also has interactive mode and you can navigate anywhere within the application